Kinetics of nitroxide reduction/reoxidation is a sensitive method of investigation of redox processes in biological and artificial systems. Essentially, it gives a new type of information not available from "classical" spin-probe methods based on EPR spectra analysis. In this project kinetics of nitroxide reduction and spin-trapping reactions will be used to characterize: 1. the concentration and reactivity of some reducing agents in multi-component solutions of microbiological origin, and 2. transmembrane diffusion of nitroxides in supported liquid biomimetic membranes and its dependence on rotational correlation times, calculated from EPR spectra.